The purpose of this project is to determine the effect of aging on cell membrane receptors, function, and integrity. Parameters of membrane integrity were studied using cells from young or old mice before and after a Sendai infection of the GRC animal colony. It was found that: (1) cells from old mice were more fragile than those from young mice before the Sendai infection, and (2) that Sendai infection increased the frequency of cell death at both 4 degrees C and 37 degrees C; however, the frequency of cell death in cell populations from old mice was 50 percent higher than in those from young mice. Lymphocytes from elderly humans were found to be more fragile than those from young individuals.